Do not crash ns_rebalance_observer:get_detailed_progress...

... when rebalance fails. Because it causes 500 response on /tasks
call and therefore ugliness in ui

Change-Id: I4aa2364009f5ad9200a1d5a2bd202a637abf869f
Reviewed-on: http://review.couchbase.org/35741
Tested-by: Artem Stemkovski <artem@couchbase.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>